Calibrated mechanical orthopedic driver with wear-compensated torque-limiting mechanism

ABSTRACT

A torque-limiting driver for orthopedic surgical use is described. The torque-limiting driver has a housing to which a torque force may be applied and transferred to a driver output shaft. The housing encloses a cam bearing assembly having a ball cage disposed so that balls of the ball cage move along a first vector path (Vr) radial to the axis of rotation. The bearing assembly also has an inner race abutting the ball. The inner race travels along a second vector path (F) parallel to the axis of rotation. The first vector path (Vr) and the second vector path (F) are not co-axial. A bearing load assembly applies a bias force (F) to the inner race of the cam bearing assembly to set the calibrated maximum amount of torque that can be transmitted via the housing through the cam bearing assembly to the output shaft of the torque-limiting driver.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of prior filed U.S.Provisional Application Ser. No. 60/870,455, filed on 18 Dec. 2006,which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is in the field of orthopedic surgicalinstrumentation (believed to be classified in US class 606/53).Specifically, the present invention relates to surgical instrumentationfor use in bone preparation for the manipulation, placement or removalof an internal bone prosthesis (believed to be classified in US class606/53; 86). More specifically, the present invention relates to a screwor pin placement or removal means particularly adapted for use in anorthopedic environment for inserting or extracting an elongated elementhaving helical threads (believed to be classified in US class 606/53;86; 104).

BACKGROUND OF THE INVENTION

When a mechanical fastener driver is used to insert or to remove athreaded fastener, rotational force or torque is applied to the fastenerto cause it to rotate. In this manner, the fastener can be driven intoor removed from a work piece. In the orthopedic surgical arts, the workpiece is usually bone. There exist in the orthopedic surgical artsapplications in which threaded fasteners are inserted into and removedfrom bone. As in other fields, there exists the need in some of theseapplications to control the torque applied via the driver to thefastener. For example in the orthopedic surgical arts, it is common fora threaded fastener to be driven into a human bone. A universal problemin the field is that when the torque applied to the driver is too great,the bone at the work site may be permanently damaged by the fastener.Also, where one surgeon may successfully drive the screw into the worksite, a different surgeon or the same surgeon on a different occasionmay apply too great a force to the fastener, damaging the bone.Additionally, in some surgical procedures, if the fasteners are set withinsufficient torque, this can result in a bad outcome as well.

Thus, there is a continuing need in the orthopedic surgical field formechanical drivers adapted for specific surgical applications in whichthe torque transmitted via the driver to the orthopedic fastener iscontrolled such that different operators of the driver cannot exceed apredetermined torque when using a driver for that application. The fieldhas been motivated to address this need and torque-limiting drivers areavailable for orthopedic use.

However, a continuing problem in the industry is that, although thecalibration of such instruments can be accurately set during theirproduction, once in use in the field, their repeated use, cleaning andsterilization (heat and chemical) gradually alters the calibrationsetting of these instruments and shortens their useful service life. Itwould be beneficial in the orthopedic surgery industry to have availablean alternative calibrated torque-limiting fastener driver adapted fororthopedic surgical use that has an extended accurate calibrationservice life.

SUMMARY OF THE INVENTION

The present invention is a calibrated mechanical torque-limiting driverfor orthopedic surgical use. The present torque limiting-driver has awear-compensated torque-limiting mechanism that substantially increasesthe durability of the pre-set torque calibration beyond other currentlyavailable orthopedic torque-limiting drivers. The driver limits themaximum amount of rotational force, or torque, transferable to thedevice's driver output shaft. In keeping with its orthopedic instrumentfeatures and limitations, the driver is adapted to permit its cleaningand sterilization between uses. The present mechanical torque-limitingorthopedic fastener driver comprises a housing assembled of at least twomain parts: a first proximal (the user) housing section and a seconddistal housing section, which also serves as a torque setting cap. Thefirst proximal housing section has a drive end at which a driveinterface is disposed. In a preferred embodiment, the driver interfaceis attached to a manual T-handle. The second housing section has adistal shaft end with a shaft port through which the driver output shaftof the driver device extends. The first and the second housing sectionsare mechanically linked with each other via a coupling that fastens thehousing sections together.

The wear compensating, torque-limiting assembly of the driver isdisposed within the housing. The torque limiting assembly mechanicallyconnects the housing and drive interface with the driver output shaft.The torque-limiting assembly is finely adjustable to selectively set themaximum amount of torque that can be transmitted via the drive interfaceof the housing to the driver output shaft. This is accomplished via atorque adjustment mechanism portion of the torque-limiting assembly. Thedriver output shaft has a housing end received and freely rotatable inthe first housing section. A shank portion of the driver output shaft isin communication with the torque-limiter assembly, and rotatabledepending on the amount of torque being applied to the housing. Theoutput shaft extends from a shaft port at the distal shaft end of thesecond housing section. The driver output shaft has a distal fastenerinterface end adapted to engage an orthopedic fastener, such as a bonescrew or an extension device. Because there are many differentconfigurations of orthopedic fasteners, the fastener interface end canbe set up to accept an adaptor which mates with a specific configurationof fastener, or alternatively, because the output shaft itself is easilyremovable and replaceable, different output shafts can be provided whichhave their distal fastener interface end specifically adapted for usewith a desired fastener.

The coupling means for joining the housing components can beaccomplished by any of a variety of means know to and selectable by oneof skill in the art, so long as the means allows disassembly andreassembly of the housing sections to provide access to thetorque-limiting assembly. Additionally, the torque-limiting assembly isadapted to provide for cleaning and sterilization between uses.

In particular, the torque-limiting assembly of the present driveraddresses the need in the orthopedic surgical industry for a calibratedtorque-limiting fastener driver, wherein the calibrated maximum torquesetting remains appropriately correct despite the expected wear ofbearing surfaces and change in the physical constants of biasingcomponents, in order to extend the accurate calibration service life ofthe instrument.

The torque-limiting assembly was designed to easily and finely set thecalibration of the torque limitation of the present device, and theassembly cooperates with the housing coupling to provide this feature inthe present driver. However, an unexpected result of the design of thetorque-limiting assembly is that the service life expectancy of thecalibration setting is substantially increased. This unexpected resultaddresses a continuing problem in the industry in that, although thecalibration of an orthopedic driver can be accurately set duringproduction, once in use in the field, repeated use, cleaning andsterilization (heat and chemical) gradually alter the calibrationsetting of the instrument and shorten its useful service life.

There are three main component features of an orthopedic mechanicaltorque-limiting device that are subject to wear and that canconsequently cause loss of calibration over time from repeated usage andsterilization. These are: the two main load bearing surface contactinterfaces, and the biasing mechanism. Although there are other pointsof wear in the device, these are the ones that typically have thegreatest influence on loss of calibration. More specifically, thesecomponent features are: (1) the point load interface between each of themain bearing balls and the outer bearing race; (2) the point loadinterface between the main bearing balls and the inner bearing race; and(3) the counter-torque bias spring. The first two are surface-to-surfacewear problems. The third problem is a change in spring tension (thenormal bias force) exhibited by the bias spring due to normal use, andalso in part due to the effect of repeated sterilization of the device,especially heat sterilization. The wear-compensating design of thepresent torque-limiting assembly solves this problem by distributing oneof the points of wear over a very much larger contact surface, and byusing the other point of wear to alter a force vector to compensate forchange in the Hook's constant (or its equivalent) of the bias spring.The specifics of wear-compensation mechanism will be detailed below.

The torque-limiting assembly includes a bias mechanism, which applies aloading force to a dome-shaped inner bearing race. The shaped inner racetransmits pressure to a set of departured ball bearings disposed in anouter cam race with a lobulated race profile/surface. The cage of thedepartured balls is fixed to the output shaft of the driver. When torqueis applied to the driver interface, the balls tend to engage the detentlobes on the race surface of the profiled cam race and rotate with thecam, thus rotating the cage and attached driver output shaft. Sufficienttorque causes the balls to roll up the slope of the detent lobes. Whenthe balls pass the high point of the detent lobes on the cam race, thecam race slips (the balls advance to the adjacent detent lobe) relativeto the cage and rotation is not imparted to the driver output shaft. Themaximum torque of the torque-limiting driver may be controlled byadjusting the second housing cap of the device. The relationship betweenthe structure and function of these elements and features are made clearto one of ordinary skill in the art in view of the detailed descriptionbelow and the drawings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are side and front plan views of a torque-limitingorthopedic fastener driver of the present invention.

FIG. 2 is a front plan with enlarged partial cross-sectional view of anembodiment of the present orthopedic fastener driver.

FIGS. 3A and 3B are exploded views showing the relationship of the majorcomponents of the present orthopedic fastener driver.

FIG. 4A is a cross-sectional view taken through torque limiter andhousing of the fastener driver along the plane indicated in FIG. 1A.

FIG. 4B is a top plan view of a portion of an outer cam raceexemplifying an inner cam surface having symmetric cam lobes.

FIG. 4C is a cross-sectional view of an example of a cam bearingassembly taken along the plane indicated in FIG. 1A.

FIG. 4D is a partial cross-sectional view of a proximal portion of thedriver output shaft of the present orthopedic fastener driver.

FIG. 4E is a cross-sectional view of the first/proximal drive endhousing taken from the enlarged portion of FIG. 2.

FIGS. 5A and 5B are top plan views of a portion of an outer cam raceexemplifying an inner cam surface having asymmetric cam lobes, andshowing (A) new or unworn lobes and (B) worn lobes.

FIG. 6 is a cross-sectional view taken through the torque-limitingassembly of the fastener driver taken from the enlarged portion of FIG.2.

FIGS. 7A-7C are cross-sectional views taken through different portionsof the fastener driver in a normal condition: (A) cam bearing assemblytaken from FIG. 4A, (B) the torque limiting assembly, the bearing loadassembly and output shaft taken from the enlarged portion of FIG. 2, and(C) the torque-limiting assembly taken from FIG. 6.

FIGS. 8A-8C are cross-sectional views taken through different portionsof the fastener driver in a torque-loaded condition: (A) cam bearingassembly taken from FIG. 4A, (B) the torque-limiting assembly, thebearing load assembly and output shaft taken from the enlarged portionof FIG. 2, and (C) the torque-limiting assembly taken from FIG. 6.

FIGS. 9A and 9B are cross-sectional views taken through a portion of thecam bearing assembly showing the relationship of the ball bearing to theinner race when the assembly is new and the lobes of the cam race (notshown) are unworn.

FIGS. 10A and 10B are cross-sectional views taken through a portion ofthe cam bearing assembly showing the relationship of the ball bearing tothe inner race when the assembly is worn and the lobes of the cam race(not shown) are worn.

FIGS. 11A and 11B are schematic drawings illustrating the mathematicalrelationship of the force vectors at play in the present wearcompensated torque-limiting driver.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, the details of preferred embodiments ofthe present invention are graphically and schematically illustrated.Like elements in the drawings are represented by like numbers, and anysimilar elements are represented by like numbers with a different lowercase letter suffix. In the following detailed description, reference ismade to the accompanying drawings, which show by way of illustrationspecific embodiments in which the invention may be practiced. However,it is to be understood that other embodiments will become apparent tothose of ordinary skill in the art upon reading this disclosure. Thefollowing detailed description is, therefore, not to be construed in alimiting sense, as the scope of the present invention is defined by theclaims.

As illustrated in FIGS. 1A and 1B, the present invention is a wearcompensated torque-limiting driver 10 suitable for orthopedic surgicaluse. The driver 10 comprises a housing 16 to which a torque force may beapplied, for example, via a manually operated T-handle 12. As shown inFIG. 2, the housing 16 encloses a torque limiting assembly 50 includinga cam bearing mechanism 52 having a ball cage 54 and ball bearings 56.The ball cage 54 is disposed so that at least one ball bearing 56 of theball cage 54 moves along a first radial direction or path Vrperpendicular to the axis of rotation 20 of the driver 10 due to atorque applied to the housing. An inner bearing race 58 abuts the atleast one ball bearing 56, which inner race 58 travels along a seconddirection or path Tr while abutting the at least one ball bearing 56.The first direction Vr and the second Tr are not co-axial. In theembodiment exemplified in the drawings, the first Vr and second Trdirections are at a right angle to each other. A bearing load assembly70 is in mechanical communication with the cam bearing assembly 52 andapplies a normal bias force F to the inner race 58 of the cam bearingmechanism 52. In the preferred embodiment, this bias is selectively setduring production of the driver to set the calibrated maximum amount oftorque that can be transmitted via the housing 16 through the cambearing assembly 52 to a driver output shaft 18 of the torque-limitingdriver 10. The driver 10 is adapted to enable its cleaning andsterilization for orthopedic surgical use.

The present wear-compensated, calibrated mechanical torque-limitingorthopedic fastener driver 10 is adapted for surgical use. It can alsobe subjected to the sterilization processes typical for such instrumentsin the field. As illustrated in FIGS. 3A and 3B, the driver 10 comprisesa housing 16 having a first proximal housing section 22 with a drive end24, at which a drive interface 14 is disposed. A second housing section26 mechanically communicates with the first housing section 22 by ahousing coupling 40. In the embodiment illustrated, the housing coupling40 comprises complementary threaded interfaces 41, 42 on the first andthe second housing sections 22, 26 of the housing 16. More specifically,the housing coupling 40 comprised an externally threaded interface 41 onthe first housing section 22 of the housing 16, and a complementaryinternally threaded interface 42 on the second housing section 26. Thesecond housing section 26 has a distal shaft end 28 with a shaft port 30through which the driver output shaft 18 projects along the rotationaxis 20 of the fastener driver 10. FIG. 4A is a cross-sectional viewtaken through torque limiter and housing of the fastener driver alongthe plane indicated in FIG. 1A and illustrates the relationships of thehousing sections 22, 26 to the cam bearing assembly components 56, 58,60 and the output shaft 18.

A wear-compensated torque-limiting assembly 50 is disposed within thehousing 16. The wear-compensated torque-limiting assembly 50 ismechanically adapted to apply torque from the drive interface 14 to thedriver output shaft 18. This is accomplished via a cam bearing assembly52 which converts rotational force on the housing 16 into a radial forcevector Vr on the ball hearings 56 of the cam bearing assembly 52. Forreference, see FIG. 4C and FIG. 6. The torque-limiting assembly 50 incombination with the bearing load assembly 70 provides for selectivelysetting a maximum amount of torque that can be transmitted by the driveinterface 14 of the housing 16 to the driver output shaft 18. Thebearing load assembly 70 is in mechanical communication with thetorque-Limiting assembly 50, and applies a normal bias force F theretoto selectively set the amount of rotational force on the housing 16 thatcan translate into the calibrated maximum amount of radial force vectorVr and that can be transmitted via the torque limiting assembly to thedriver output shaft 18.

The driver output shaft 18 has a proximal housing end 80 (FIG. 4D)passing through the torque-limiter assembly 50, which is received alongthe axis of rotation 20 in rotatable communication within the proximalhousing section 22. As shown in FIGS. 4D and 4E, this is accomplished inthe illustrated embodiments by the drive end 24 of the first housingsection 22 being adapted to have a pilot bearing receiver 84 disposedinside the drive end housing section 22 concentric to the rotation axis20. The pilot hearing receiver 84 closely receives the shaft pilotbearing 82 of the driver output shaft 18. Optionally, the pilot bearingreceiver 84 disposed inside the drive end housing section 22communicates with a housing cannula 86 disposed concentric to therotation axis 20 through the drive end 24 of the first housing section22. Preferably, as illustrated in the drawings, a thrust bearingassembly 44 is disposed in front of the pilot bearing receiver 84concentric to the rotation axis 20, through which the shaft pilot 82 ofdriver output shaft 18 passes to enter the pilot bearing receiver 84.The thrust bearing assembly 44 is in rotating communication with theball cage 54 of the cam bearing assembly 52. The driver output shaft 18optionally has a central shaft cannula 87. When the output shaft 18 isreceived by the pilot bearing receiver 84, the housing cannula 86 iscoaxially aligned with the shaft cannula 87. The output shaft 18 alsohas a driver shank portion 82, which extends through the shaft port 30of the second housing section 26. The distal shaft end 84 of the outputshaft 18 is adapted to terminate in a fastener, interface 88 forengaging a fastener, an extension rod or other tool head (not shown).

The wear-compensated torque-limiting assembly 50 comprises a cam bearingassembly 52 (FIG. 4C) having a ball cage 54 and a plurality of ballbearings 56 held in a departured relationship to each other by the ballcage 54. The cam bearing assembly 52 has an outer cam race 60 in whichthe ball bearings 56 of the ball cage 54 are received. An innerwear-dispersing ball loading race 58 is received, in turn, within theball cage 54. The outer cam race 60 is held in a fixed non-rotatingcondition relative to the housing 16. This is accomplished in theembodiments illustrated by a cam retainer interface 67 on an outersurface 66 of the outer cam race 60 received in a complementary camretainer interface 68 on an inner surface 23 of the first housingsection 22. For reference, see FIGS. 4A to 4C. The ball cage 54 is heldin a fixed non-rotating condition relative to the driver output shaft 18by the keyed cam assembly bore 53 concentric with the rotation axis 20.The keyed cam assembly bore 53 of the ball cage 54 is adapted to engagea complementary keyed (or asymmetric) interface 88 at the housing end 80of the driver output shaft 18. For reference, see FIGS. 4C and 4D. Themating of the complementary keyed interfaces 53 and 88 prevents rotationof a ball cage 54 of the cam bearing assembly 52 relative to the driveroutput shaft 18.

The cam race 60 and the inner race 58 are both in mechanicalcommunication with the ball bearings 56. The outer cam race 60 is inmechanical communication with the ball bearings 56 and moves them in adirection along a direction vector Vr substantially along a radius ofthe rotation axis 20 of the fastener driver 10. The inner bearingloading race 58 is in mechanical communication with the ball bearings 56and is moved by them in a non-radial direction substantially parallel tothe rotation axis 20 and in the direction of the bias force vector F(when torque is applied to the cam race 60 via the housing section 22).The resultant non-radial force vector Vn of the combined movementdirections is at an angle θ relative to the direction vector Vr. Forreference, see FIG. 6. FIGS. 7A-7C are cross-sectional views takenthrough different portions of the fastener driver 10 in a normal (notorque applied) condition: (A) cam bearing assembly taken from FIG. 4A,(B) the torque-limiting assembly, the bearing load assembly and outputshaft taken from the enlarged portion of FIG. 2, and (C) thetorque-limiting assembly taken from FIG. 6. FIGS. 8A-8C arecross-sectional views taken through the same portions of the fastenerdriver 10, but in a torque-loaded condition. What these figuresillustrate is that in response to sufficient torque being applied to thecam race 60 of the cam bearing assembly 52, the ball bearings 56 areforced to move along a radius of the rotation axis 20 in direction Vr.The ball bearings 56 move radially to the rotation axis 20. Movement ofthe ball bearings 56 applies a force against the wear dispersing surface92 of the inner race 58 causing it to travel a distance Ta along therotational axis 20 against the bias force F. The axial travel distanceTa of the inner race 58 is determined by the difference in the forceexerted by the ball 56 against the race surface 92 and the bias force Fon the inner race 58. This difference in force dF is equivalent: to theresultant non-radial vector force Vn. (For support, see FIGS. 11A and11B). As shown in FIGS. 9A and 9B, when the cam bearing assembly is newor the lobes 64 of the cam race 60 are unworn, the radial distance theball 56 must travel (the throw of the ball) Tb to reach the threshold ofthe lobe 65 is at a maximum. For reference, see also FIG. 5A. However,as shown in FIGS. 10A and 10B, as the lobes 64 wear, the radial distanceTb the ball 56 must travel to reach the threshold of the lobe 65 getssmaller. For reference, see also FIG. 5B.

The outer cam race 60 has an inner race surface 61 in mechanical contactwith the ball bearings 56. The inner surface 61 is adapted with aplurality of cam lobes 64 disposed to provide that each ball bearing 56is similarly accommodated in a lobe 64. There can be fewer ball bearings56 in the ball cage 54 than there are lobes 64 in the inner race surface61. Each lobe 64 has a bottom ball detent portion 62, two ramp portions63 a and 63 b and a cam lobe high-point portion 65. A ball throwdistance Tb is defined as the distance between the bottom ball detentportion 62 and the cam high-point portion 65 of the lobe 64 along aradius of the rotation axis 20.

The inner ball loading race 58 has a dome shaped portion 90 (FIG. 3B)with a wear-dispersing outer surface 92. A central bushing 94 runsthrough the inner race 58 perpendicular to a base 96 of the dome shapeportion 90 and concentric with the rotation axis 20 of the driver 10.This bushing 94 is slidable over the shank 82 of the output shaft 18along the rotation axis 20. The dome shaped outer surface 92 forms theangle θ between a radius of the rotation axis 20 and a ball radiusperpendicular to a point of contact of the ball bearing 56 with theouter surface 92. The angle θ increases at a rate dependent on thecurvature of the outer surface 92 as an axial displacement Ta increases.A specific advantage of the dome shaped outer surface 92 of the innerwear-dispersing ball loading race 58 is that it presents a substantiallylarger contact surface for mechanical contact with the ball bearings 56,and consequently disperses wear from the ball bearings 56 over asubstantially larger contact surface than with a conventional bearingrace.

The bearing load assembly 70 has a mechanism to provide a normal biasforce to the inner race 58, preferably through a thrust bearing assembly44. In the preferred embodiment illustrated, the bearing load assembly70 utilizes a set of Belleville washers to accomplish the bias mechanism72. However, one of skill in the art could select and practice otherbiasing mechanisms in the present invention, such as: a coil spring 72a, a set of Belleville washers 72 b, a gradient e.g., gas pistoncompression device, and a compression resistant material 72 c inmechanical communication with the wear-compensated torque-limitingassembly 50.

The outer cam race 60 of the mechanical torque-limiting assembly 50 hasan inner race surface 61 adapted with a plurality of cam lobes 64. Inone preferred embodiment, the inner race surface 61 has an asymmetricalprofile, which preferentially limits rotation of the driver output shaft18 to a single direction, e.g., clockwise. For reference, see FIGS. 4Aand 4C. The asymmetry lies in one slope 63 a of the lobe 64 beingshorter than the other slope 63 b. Alternatively, the inner surface 61of the cam race 60 can be adapted with a plurality of cam lobes 64having a symmetrical profile to enable rotation of the driver outputshaft 18 in a first clockwise direction and a second counter-clockwisedirection, and the symmetrical profile limits the predetermined torqueequally in both directions of rotation. For reference, see FIG. 4B. Inthis case, the symmetry lies in both slopes 63 a, 63 b of the lobe 64being the same.

As shown in FIGS. 5A and 5B, the lobes 64 of the outer cam race 60 aresubject to wear, particularly, the cam lobe high-point portion 65. Asthe cam lobe high-point portion 65 wears, the ball throw distance Tbdecreases. For reference, see and compare FIG. 5A to FIG. 5B. This isone of the points of wear in the instrument 10 that is a potentialsource of de-calibration.

An example of the wear-compensated torque-limiting feature of thepresent driver 10 is as follows read in conjunction with FIGS. 11A and11B.

EXAMPLE 1

At the outset, it is noted that the sine of an angle is as follows: sinA=a/c, sin B=b/c. The cosine of an angle is as follows: cos A=b/c, cosB=a/c. The tangent of as angles is as follows: tan A=a/b, tan B=b/a.

Assume θ=30° and Vr=100 units when instrument is new.

A. New Cam Race and New Spring

-   -   cos θ=Vr/Vn    -   Vn=100 units/0.866=115 units    -   sin θ=F/115 units    -   F=0.500×115 units=58 units

Therefore, in this system: 58 units of Bias force (F) equal 100 units ofTorque force (Vr)

B. Effect of Worn Spring

-   -   Assume Bias force (F) reduced by 10%.    -   F=52 units    -   What is the equivalent torque force (Vr)?    -   Tan θ=F/Vr    -   VR=52 units/0.577=90        C. However: Effect of Worn Cam Race Lobe    -   Assume θ=27° due to cam wear    -   Assume Bias force (F) reduced by 10%: F=52 units    -   What is the equivalent Torque force (Vr)?    -   Tan 27′=F/Vr    -   Vr=52 units/0.510=102 units

Therefore, in this scenario, a 10% reduction in Bias force (F) and a 10%reduction in θ due to cam lobe wear do not substantially altercalibration of the torque (Vr) of the instrument.

In an advantage, the torque-limiting driver 10 may be used in anapplication in which precision torquing operations are performed. As oneexample, the torque-limiting driver 10 may be used in surgicaloperations in which screws are driven into a bone, such as duringorthopedic operations and the like. By controlling the torque applied tothe screw, the torque-limiting driver 100 ensures that, no matter whichsurgeon drives the screw into the bone, the screw will be driven at apredetermined torque.

While the above description contains many specifics, these should not beconstrued as limitations on the scope of the invention, but rather asexemplifications of one or another preferred embodiment thereof. Manyother variations are possible, which would be obvious to one skilled inthe art. Accordingly, the scope of the invention should be determined bythe scope of the appended claims and their equivalents, and not just bythe embodiments.

The invention claimed is:
 1. A torque-limiting fastener driver, whichcomprises: a) a housing comprising a proximal housing section having adrive end at which a drive interface is disposed, and a distal housingsection connected to the proximal housing section by a housing coupling,wherein the distal housing section has a shaft port into which a driveroutput shaft is detachably insertable to extend along a rotation axis ofthe fastener driver; b) a torque-limiting assembly disposed within thehousing, the torque-limiting assembly comprising: i) a cam bearingassembly comprising at least two ball bearings held in a departuredrelationship to each other by a ball cage, and ii) an outer cam race inwhich the ball bearings are received, wherein the outer cam race has aninner race surface provided with a plurality of repeating asymmetric camlobes, each asymmetric cam lobe comprising a cam high point spaced froma bottom ball detent portion intermediate opposed ramp portions of alesser and a greater pitch, and iii) an inner race comprising adome-shaped outer surface aligned symmetrically along the rotation axis,iv) wherein the outer cam race contacts each ball bearing received in alobe at a radial vector (Vr) that is aligned substantially along aradius perpendicular to the rotation axis of the fastener driver, andthe outer surface of the inner race contacts each ball bearing at anon-radial vector off-set by an angle from the radial vector; and c) abearing load assembly providing a bias force (F) against thetorque-limiting assembly, wherein the bias force is adjustable toselectively set a maximum amount of torque that is transmittable via thedrive interface to the driver output shaft that is detachably insertableinto the shaft port at the distal section of the housing, d) wherein ina rest position of the driver, the ball bearings reside in the bottomball detent portion of a first cam lobe of the outer cam race and incontact with the inner race, and e) wherein with the driver output shaftreceived in the shaft port at the distal housing section, the drive endof the proximal housing section is manipulatable to impart a torqueforce that causes the inner race to rotate about the rotation axis asthe ball bearings contacting the outer cam race at the radial vectormove up the lesser pitch ramp of one of the plurality of asymmetric camlobes while contacting the outer surface of the inner race at thenon-radial vector to thereby axially displace the inner race in a firstdirection along the rotation axis until the ball bearings crest at thecam high point and fall along the greater pitch ramp into the bottomball detent portion of a next one of the plurality of asymmetric camlobes with the inner race then moving in a second, opposite directionalong the rotation axis back to the rest position.
 2. The driver ofclaim 1 wherein the housing coupling comprises complementary threadedinterfaces on the proximal and distal housing sections.
 3. The driver ofclaim 1 wherein the housing coupling comprises a first externallythreaded interface on the proximal housing section and a complementaryinternally threaded interface on the distal housing section.
 4. Thedriver of claim 1 wherein the drive end of the proximal housing sectionhas a pilot bearing receiver disposed therein that is concentric withthe rotation axis, the pilot bearing receiver being sized to receive ashaft pilot of the driver output shaft.
 5. The driver of claim 4 whereinthe pilot bearing receiver includes a housing cannula that is concentricwith the rotation axis of the drive end of the proximal housing section.6. The driver of claim 4 wherein a thrust bearing assembly disposeddistal to the pilot bearing receiver and concentric to the rotation axisis rotatable with the ball cage of the cam bearing assembly, and whereinthe shaft pilot of the driver output shaft which is detachablyinsertable into the shaft port of the distal housing section passesthrough the pilot bearing receiver.
 7. The driver of claim 1 wherein theouter cam race is in a fixed, non-rotating relationship relative to thehousing.
 8. The driver of claim 1 wherein a cam retainer interface on anouter surface of the outer cam race is held in a fixed, non-rotatablerelationship with a complementary cam retainer interface on an innersurface of the proximal housing section.
 9. The driver of claim 8wherein the asymmetrical profile of the cam lobes on the inner racesurface of the outer cam race limit rotation of the driver output shaftdetachably inserted into the shaft port of the distal housing section toa single direction.
 10. The driver of claim 1 wherein the ball cage hasa cam assembly bore concentric with the rotation axis of the housing,the cam assembly bore being configured to engage an asymmetric interfaceof the driver output shaft to prevent rotation of the ball cage of thecam bearing assembly relative to the driver output shaft.
 11. The driverof claim 1 wherein a central bushing runs through the inner race,concentric with the rotation axis and slidable over a shank of thedriver output shaft.
 12. The driver of claim 1 wherein the bearing loadassembly comprises a biasing device selected from the group consistingof: a coil spring, a set of Belleville washers, a gradient compressiondevice, a gas piston, and a compression resistant material in mechanicalcommunication with the torque-limiting assembly.
 13. The driver of claim1 wherein the bearing load assembly is in mechanical communication witha thrust bearing assembly biasing against the torque-limiting assembly.14. The driver of claim 1 further comprising a handle disposed at thedrive interface of the proximal housing section.
 15. The driver of claim1 wherein a ball throw distance (Tb) is defined as the distance betweenthe bottom ball detent portion intermediate the opposed ramp portionsand the can high-point portion of one of the lobes along a radius of therotation axis.
 16. The driver of claim 1 wherein the torque-limitingassembly is adjustable to selectively transmit a torque (Vr) from thedrive interface of the housing to the driver output shaft that isdetachably insertable into the shaft port at the distal end of thehousing.
 17. The driver of claim 1 wherein the outer surface of theinner race forms an angle between the radial vector (Vr) and a ballradius perpendicular to a point of contact of the ball bearing with theouter surface of the inner race, the angle increasing as the inner raceis axially displaced in the first direction along the rotation axis as aresult of the torque force imparted to the driver.
 18. A torque-limitingfastener driver, which comprises: a) a housing comprising a proximalhousing section having a drive end at which a drive interface isdisposed, and a distal housing section connected to the proximal housingsection by a housing coupling, wherein the distal housing section has ashaft port into which a driver output shaft is insertable to extendalong a rotation axis of the fastener driver; b) a torque-limitingassembly disposed within the housing, the torque-limiting assemblycomprising: i) a cam bearing assembly comprising at least two ballbearings held in a departured relationship to each other by a ball cage,ii) an outer cam race in which the ball bearings are received, whereinthe outer cam race has an inner race surface provided with a pluralityof asymmetric cam lobes, each asymmetric cam lobe comprising a cam highpoint spaced from a bottom ball detent portion intermediate opposed rampportions of a lesser and a greater pitch, and iii) an inner racecomprising a dome shaped portion with an outer surface alignedsymmetrically along the rotation axis, iv) wherein the outer cam racecontacts each ball bearing received in a lobe at a radial vector (Vr)that is aligned substantially along a radius perpendicular to therotation axis of the fastener driver, and the outer surface of the innerrace forms an angle between the radial vector (Vr) and a ball radiusperpendicular to a point of contact of the ball bearing with the outersurface of the inner race, the angle increasing as an axial displacement(Ta) of the inner race increases as a result of a torque force impartedto the driver, c) wherein in a rest position of the driver, the ballbearings reside in the bottom ball detent portion of a first cam lobe ofthe outer cam race and in contact with the inner race, and d) whereinwith the driver output shaft received in the shaft port at the distalhousing section, the drive end of the proximal housing section ismanipulatable to impart a torque force that causes the inner race torotate about the rotation axis as the ball bearings contacting the outercam race at the radial vector move up the lesser pitch ramp of one ofthe plurality of asymmetric cam lobes while contacting the outer surfaceof the inner race at a non-radial vector to thereby axially displace theinner race in a first direction along the rotation axis as the angleincreases until the ball bearings crest at the cam high point and fallalong the greater pitch ramp into the bottom ball detent portion of anext one of the plurality of asymmetric cam lobes with the inner racethen moving in a second, opposite direction along the rotation axis backto the rest position.
 19. The driver of claim 18 wherein a bearing loadassembly provides a bias force (F) against the torque-limiting assembly.20. A torque-limiting fastener driver, which comprises: a) a housingcomprising a proximal housing section having a drive end at which adrive interface is disposed, and a distal housing section connected tothe proximal housing section by a housing coupling, wherein the distalhousing section has a shaft port into which a driver output shaft isdetachably insertable to extend along a rotation axis of the fastenerdriver; b) a torque-limiting assembly disposed within the housing, thetorque-limiting assembly comprising: i) a cam bearing assemblycomprising at least two ball bearings held in a departured relationshipto each other by a ball cage, ii) an outer cam race in which the ballbearings are received, wherein the outer cam race, which is in a fixed,non-rotating relationship relative to the housing, has an inner racesurface provided with a plurality of repeating asymmetric cam lobes,each asymmetric cam lobe comprising a cam high point spaced from abottom ball detent portion intermediate opposed ramp portions of alesser and a greater pitch, and iii) an inner race comprising a domeshaped portion with an outer surface aligned symmetrically along therotation axis, iv) wherein the outer cam race contacts each ball bearingreceived in a lobe at a radial vector (Vr) that is aligned substantiallyalong a radius perpendicular to the rotation axis of the fastenerdriver, and the outer surface of the inner race forms an angle betweenthe radial vector (Vr) and a ball radius perpendicular to a point ofcontact of the ball bearing with the outer surface of the inner race,the angle increasing as an axial displacement (Ta) of the inner raceincreases as a result of a torque force imparted to the driver, c)wherein in a rest position of the driver, the ball bearings reside inthe bottom ball detent portion of a first cam lobe of the outer cam raceand in contact with the inner race, and d) wherein with the driveroutput shaft received in the shaft port at the distal housing section,the drive end of the proximal housing section is manipulatable to imparta torque force that causes the inner race to rotate about the rotationaxis as the ball bearings contacting the outer cam race at the radialvector move up the lesser pitch ramp of one of the plurality ofasymmetric cam lobes while contacting the outer surface of the innerrace at a non-radial vector to thereby axially displace the inner racein a first direction along the rotation axis as the angle increasesuntil the ball bearings crest at the cam high point and fall along thegreater pitch ramp into the bottom ball detent portion of a next one ofthe plurality of asymmetric cam lobes with the inner race then moving ina second, opposite direction along the rotation axis back to the restposition.
 21. The driver of claim 20 wherein a bearing load assemblyprovides a bias force (F) against the torque-limiting assembly.
 22. Thedriver of claim 21 wherein the bias force is adjustable to selectivelyset a maximum amount of torque transmitted via the drive interface tothe driver output shaft that is detachably insertable into the shaftport at the distal section of the housing.
 23. A torque-limitingfastener driver, which comprises: a) a housing comprising a proximalhousing section having a drive end at which a drive interface isdisposed, and a distal housing section connected to the proximal housingsection by a housing coupling, wherein the distal housing section has ashaft port into which a driver output shaft is detachably insertable toextend along a rotation axis of the fastener driver; b) atorque-limiting assembly disposed within the housing, thetorque-limiting assembly comprising: i) a cam bearing assemblycomprising at least two ball bearings held in a departured relationshipto each other by a ball cage, and ii) an outer cam race in which theball bearings are received, wherein the outer cam race has an inner racesurface provided with a plurality of repeating asymmetric cam lobes,each asymmetric cam lobe comprising a cam high point spaced from abottom ball detent portion intermediate opposed ramp portions of alesser and a greater pitch, and iii) an inner race comprising adome-shaped outer surface aligned symmetrically along the rotation axis;and c) a bearing load assembly providing a bias force (F) against thetorque-limiting assembly, wherein the bias force is adjustable toselectively set a maximum amount of torque that is transmittable via thedrive interface to the driver output shaft that is detachably insertableinto the shaft port at the distal section of the housing, d) wherein ina rest position of the driver, the ball bearings reside in the bottomball detent portion of a first cam lobe of the outer cam race and incontact with the inner race, and e) wherein with the driver output shaftreceived in the shaft port at the distal housing section, the drive endof the proximal housing section is manipulatable to impart a torqueforce that causes the inner race to rotate about the rotation axis asthe ball bearings contacting the outer cam race at the radial vector(Vr) aligned substantially along a radius perpendicular to the rotationaxis of the fastener driver to move up the lesser pitch ramp of one ofthe plurality of asymmetric cam lobes while contacting the outer surfaceof the inner race at a non-radial vector to thereby axially displace theinner race in a first direction along the rotation axis until the ballbearings crest at the cam high point and fall along the greater pitchramp into the bottom ball detent portion of a next one of the pluralityof asymmetric cam lobes with the inner race then moving in a second,opposite direction along the rotation axis back to the rest position.